In recent years, higher integration, higher functionality, and higher density mounting of semiconductors, which are widely used in electronic equipment, communication instruments, personal computers, and the like, have been increasingly accelerated, and the demand for the properties of printed wiring boards has increased more than ever before. Particularly, heat dissipation techniques for printed wiring boards against the heat generation of semiconductors have been required. This is because with higher functionality of semiconductors, the amount of heat generated from the semiconductors has increased, and because configurations in which heat accumulates easily inside have been provided due to the influence of higher integration and higher density mounting.
Generally, thermosetting resins, such as epoxy resins, used for the insulating layers of printed wiring boards have low thermal conductivity in themselves. Therefore, in order to improve the thermal conductivity of printed wiring boards, a thermally conductive resin composition is proposed in which 80 to 95% by weight of a mixed filler (inorganic filler) having a predetermined particle diameter distribution is blended with a thermosetting resin to attain the thermal conductivity of the cured product to 3 to 10 W/mK (see Patent Literature 1).